Recently, Homogeneous-Charge Compression Ignition (HCCI) combustion in which a gasoline fuel mixed with air is combusted by self-ignition inside a sufficiently compressed combustion chamber has attracted attention. HCCI combustion is a mode in which the mixture gas combusts at a plurality of positions simultaneously without flame propagation, and thus has a higher combustion speed of the mixture gas than in SI combustion (spark-ignition combustion) which is adopted for general gasoline engines. Therefore, HCCI combustion is said to be significantly advantageous in terms of thermal efficiency. However, in a case of implementing HCCI combustion in an engine of an automobile for which improved thermal efficiency is desired, there are various issues to be solved and an engine which suitably performs HCCI combustion has not been put into practical use. That is, while the engine mounted on the automobile greatly changes in operating state and its environmental condition, HCCI combustion has issues such as a combustion start timing of the mixture gas (a timing at which the mixture gas self-ignites) greatly varies due to external factors (e.g., atmospheric temperature) and control during a transient operation in which an engine load sharply changes being difficult.
Therefore, instead of combusting all of the mixture gas by self-ignition, it is proposed to combust a portion of the mixture gas by spark-ignition using a spark plug. That is, after forcibly combusting a portion of the mixture gas through flame propagation caused by spark-ignition (SI combustion), the remaining mixture gas is combusted by self-ignition (CI combustion). Hereinafter, such combustion mode is referred to as “SPCCI (SPark Controlled Compression Ignition) combustion.”
For example, JP2009-108778A discloses an engine adopting a similar concept to the SPCCI combustion. This engine causes flame propagation combustion by spark-igniting a stratified mixture gas which is formed around a spark plug by a supplementary fuel injection, and then performs a main fuel injection inside a combustion chamber warmed up by an effect of the flame propagation combustion, so as to combust through self-ignition the fuel injected in the main fuel injection.
The CI combustion of the SPCCI combustion occurs when a temperature inside a cylinder (in-cylinder temperature) reaches an ignition temperature of the mixture gas determined by a composition of the mixture gas. Fuel efficiency is maximized by causing the CI combustion when the in-cylinder temperature reaches the ignition temperature near a top dead center of compression stroke. The in-cylinder temperature increases as pressure inside the cylinder (in-cylinder pressure) increases. An increase of the in-cylinder pressure on the compression stroke when the SPCCI combustion is performed is caused by two factors: a pressure increase due to a compressing action of a piston and a pressure increase due to heat from the SI combustion. The pressure increase due to the SI combustion becomes more significant as a combustion speed, i.e., a flame propagation speed, is higher. When an amount of burnt gas remaining inside the cylinder (residual gas amount) is large, the flame propagation speed becomes slower since the burnt gas, which is also inert gas, interrupts the flame propagation. Therefore, the pressure increase due to the SI combustion is smaller as the residual gas amount is larger. Thus, in order to control the pressure increase due to the SI combustion so that the CI combustion occurs near TDC of compression stroke, it is considered to increase the combustion speed of the SI combustion by reducing the residual gas amount as an engine load is lower where a fuel amount is smaller and a heat generation amount inside the cylinder is smaller.
On the other hand, when the fuel amount is large and the heat generation amount inside the cylinder is large, that is when the engine load is high, a temperature of exhaust gas, i.e., the temperature of the residual gas, increases. Therefore, on the high engine load side, the residual gas amount at high temperature is increased to raise the temperature inside the cylinder and improve stability of the SI combustion.
Such an adjustment of the residual gas amount can be considered to be achieved by advancing an open timing of an intake valve on exhaust stroke as the engine load increases (or retarding the open timing as the engine load decreases). For this reason, the present inventors have intensively studied about the open timing of the intake valve. As a result, it was found that by simply advancing the open timing of the intake valve on the exhaust stroke as the engine load increases, another issue arises that excessive combustion noise is generated when the engine load is high. Therefore, to put the SPCCI combustion into practical use, the open timing of the intake valve needs to be controlled more suitably.